While open source software has existed for DOS and Microsoft Windows practically since the beginning, and some of it was relatively popular among people, most of the software available for these platforms has been non-open-source binary-only software, a lot of it from Microsoft.
This has started to change recently. The Firefox browser from the home of the Mozilla Foundation (and now also the Mozilla Corporation), is an open-source, modern and sophisticated browser, that has been virally publicised by various means such as the various "Spread Firefox" campaigns. It has become popular and as of July 2006 has passed the 10% usage in web site hits according to some firms, and in some countries much more so. It is still gaining some market share, even if it's growth has declined somewhat.
Other cross-platform open source software includes OpenOffice.org, a powerful and usable office productivity software for Windows, Linux and other platforms, the GIMP (GNU Image Manipulation Program), a sophisticated raster image editing program, and Inkscape, a vector editing program, and also most Peer-to-Peer networking clients. These have probably seen less popularity than Firefox, but are still providing cheap, open, modifiable alternatives to traditional binary-only software.
In 2003, a study was published that estimated that by 2004, more developers will work on Linux than on Windows. While it definitely does not mean that more people will use Linux at home, it is still a good indication for its general mainstream acceptance and usefulness.
Another important recent trend was the rise of open content. The first edition of this article, included a small section about "open content", where I concluded by saying that "Only time can tell whether other elements of open source besides its freely distributable nature will have an impact in other areas of creative arts besides software.". Now, about 3 years later, I can say that by all means open content has already proven to be a great success.
Among the landmarks of open or semi-open content are:
The Creative Commons project that specifies licences for open content, semi-open content or just freely redistributable artworks for individuals and organisations to use in their artwork, as well as supplying several resources for facilitating their publishing and use.
Creative Commons licences have proven to be very popular among many web publishers for use in their works.
The Wikimedia Foundation publishes several online multi-lingual wikis - web sites that are editable by the surfers - all under an open content licence. The most famous and important one are the Wikipedias, which are free, online encyclopaedias. The English Wikipedia (which is still the largest) is larger than Encyclopaedia Britannica and Microsoft Encarta combined and is growing rapidly.
There are many sites for independent musicians, such as GarageBand.com, ccMixter, and Magnatune (a record label that publishes artists whose songs are under a freely redistributable licence).
From weblogs and weblog comments, to wikis, to audiocasts or video-blogs - open or semi-open content is everywhere.